More than half of the current HIV infections in the world are due to HIV-1 C, a genotype that deserves more thorough analysis. The most dramatic epidemic of HIV-1 C is currently expanding in countries such as Botswana. It has been estimated that 38.5 percent of all pregnant women, including almost half of the pregnant women between the age 20 and 29, in Botswana are infected. Because Botswana is a democratic country with a well organized health care system, it represents an ideal partner country for vaccine development efforts aimed at curbing the expanding HIV-1 epidemic in Southern Africa. The overall goal of this proposal is to test the hypothesis that, despite the unusually high degree of sequence divergence found in the HIV-1 subtype C viruses circulating in Botswana, conserved CTL epitopes that match the most common HLA types of Batswana (note: Botswana people are called Batswana) can be identified for HIV-1 C vaccine design. The first Specific Aim is to determine the frequency of the most common HLA types in Botswana. The methodology of genetic characterization will be adopted to achieve this objective. The second Specific Aim is the molecular characterization of HIV-1 C field isolates. The objective of this specific aim is to critically assess the extent of sequence divergence and to identify regions of high sequence homology. The last Specific Aim is to identify CTL epitopes located in the conserved regions of HIV-1 C genome. Particular emphasis will be placed on the identification of functional CTL epitopes that match the most common HLA types for Batswana. Most of the CTL epitopes identified thus far have been based largely on HIV-1 subtype B sequences and the HLA types of Caucasians. This proposed research would allow identification of HIV-1 CTL epitopes most relevant for the design of vaccines for at-risk populations in Southern Africa. Furthermore, the tetrameric complexes constructed for functional characterization of CTL epitopes would become valuable for monitoring post-immunization CTL responses in future HIV-1 vaccine trials.